1. Field of the Invention
The present invention relates to an optical compensatory sheet and a liquid crystal display provided with the optical compensatory sheet.
2. Description of Prior Art
As a display for electronic office system-devices such as a desk-top personal computer and a word processor for Japanese language, CRT (cathode ray tube) has been employed so far. Recently, a liquid crystal display (hereinafter referred to as LCD) is increasingly employed instead of the CRT because of its thin thickness, light weight and low power consumption. LCD generally has a structure that a liquid crystal cell is disposed between a pair of polarizing sheets. Most of LCD use a twisted nematic liquid crystal. Operational mode of LCD using the twisted nematic liquid crystal is roughly divided into birefringence mode and optical rotatory mode.
A super twisted nematic liquid crystal display (hereinafter referred to as STN-LCD) utilizing the birefringence mode uses a super twisted nematic liquid crystal showing a twisted angle of more than 90 degrees and having steep electro-optical characteristics. Such STN-LCD, therefore, has an advantage of giving display of a large capacity by driving in time-sharing mode. However, the STN-LCD has disadvantages such as slow response (several hundred milliseconds) and difficulty in giving satisfactory gradation on display, and therefore its display characteristics are relatively poor, as compared with display characteristics of a liquid crystal display using the known active-type elements (e.g., TFT-LCD and MIM-LCD).
In the TFT-LCD and MIM-LCD, twisted nematic liquid crystal showing a twisted angle of 90 degrees is employed for displaying an image. This is called LCD of an optically rotary mode (i.e., TN-LCD). TN-LCD display mode shows fast response (several tens of milliseconds) and high display contrast, and easily gives black-white display of high contrast. Hence, the optical rotatory mode has a number of advantages compared with the birefringence mode or other modes. However, TN-LCD has disadvantages that color or contrast on display varies depending upon viewing direction to a liquid crystal display, and its display characteristics are not comparable to display characteristics of CRT.
In order to improve the viewing angle characteristics (i.e. to enlarge the viewing angle), arrangement of a phase difference film (optical compensatory sheet) between a pair of polarizing plates and TN liquid crystal cell has been proposed in Japanese Patent Provisional Publications No. 4(1992)-229828 and No. 4(1992)-258923.
The phase difference films proposed in these Publications show no optical effect when a liquid crystal display is seen from the direction vertical to a screen of the display because phase difference in the direction perpendicular to a surface of the liquid crystal display is almost zero. However, the phase difference film serves for compensation of phase difference (depending upon wave-lengths of light) that occurs when the liquid crystal display is viewed from an oblique direction. The phase difference results in unfavorable viewing angle characteristics such as coloring and disappearance of displayed image.
Even a liquid crystal display provided with such phase difference film is still not improved particularly in coloring of a displayed image and reversing of black-and-white image when the viewing direction to the liquid crystal display is greatly inclined to height or width direction to the normal to a surface of the screen. Thus, such liquid crystal display cannot be employed instead of CRT, and further is not suitable for loading in a vehicle (e.g., motor car).
Japanese Patent Provisional Publications No. 4(1992)-366808 and No. 4(1992)-366809 disclose a difference phase film of a liquid crystal cell comprising a chiral nematic liquid crystal that an optic axis is inclined to enlarge the viewing angle. The difference phase film is composed of two liquid crystal cells, and therefore needs a complicated process for its preparation and brings about increase of its weight.
Japanese Patent Provisional Publications No. 4(1992)-113301, No. 5(1993)-80323 and No. 5(1993)-157913 disclose a phase difference film of polymer chain whose optic axis or an optical elastic axis is inclined from a surface of a liquid crystal cell. The phase difference film is prepared by slicing obliquely a uniaxial polycarbonate film, and therefore a phase difference film of a large area cannot be easily prepared according to the disclosed process. Further, the publications are for inventions with regard to STN-LCD and therefore give no teachings as to the use with TN-LCD.
Japanese Patent Provisional Publication No. 5(1993)-215921 discloses the use of a birefringence plate comprising a pair of supports and a rod-like compound showing liquid crystal property. The rod-like compound showing liquid crystal property is prepared by interposing and curing the compound to compensate difference phase of LCD. However, the birefringence plate has the same structure as one of the two liquid crystal cell as mentioned above, and therefore needs a complicated process for its preparation, which is not suitable for mass production. Further, its structure brings about increase of weight. Furthermore, the publications are for inventions with regard to STN-LCD and therefore give no teachings as to the use for TN-LCD.
Japanese Patent Provisional Publications No. 3(1991)-9326 and No. 3(1991)-291601 disclose an optical compensatory sheet for LCD which is prepared by coating a solution of a polymer showing liquid crystal property on an orientation layer provided on a support film. However, the polymer showing liquid crystal property is not satisfactorily oriented on the orientation layer. Hence, the resulting compensatory sheet scarcely enlarges the viewing angle from all directions.
EP 0576302 A1 discloses a phase difference plate comprising a material having optical anisotropy in a flat plate in which the direction of the principal refractive index of the index ellipsoid is inclined from the normal of the surface. The disclosed phase difference plate shows great enlargement of the viewing angle compared with other known phase difference films as mentioned above. However, LCD provided with such phase difference plate is not comparable to CRT in the viewing angle characteristics.